This invention relates to nuclear reactors in general, and specifically to loop type and pool type fast breaders. More particularly, the present invention relates to a nuclear reactor of the loop type in which the space between a reactor vessel and a guard vessel disposed outside the reactor vessel is kept in a pressurized sealed state.
In a pool type reactor, the space to be sealed according to the present invention is defined between a main vessel and a safety vessel disposed outside the main vessel. However, for the sake of simplicity, the terms "reactor vessel" and "guard vessel" used hereinafter include also such main vessel and safety vessel, respectively, in the pool type reactor.
Conventionally, a guard vessel has been disposed outside a reactor vessel or tank. Since this guard vessel merely encompasses the reactor vessel, the space around the reactor vessel is kept at a pressure substantially equal to atmospheric pressure, and hence is kept open, in a sense. If, by any chance, damage to piping in a primary cooling system, or a core disassembly accident occurs in such a reactor, wide dispersion of radioactive substances would result. (Though every kind of safety measure has been taken in the reactor to prevent such an accident, it could be hypothetically or provisionally considered.)
This problem can be solved by keeping the space between the guard vessel and the reactor vessel in a sealed state. According to the sealed arrangement, dispersion of the nuclear fuel materials, fission products and coolants to the outside can be reliably prevented even if the reactor vessel or the piping in the primary cooling system disposed near the reactor vessel is damaged.
One possible method of fixing the reactor vessel and the guard vessel to each other in a sealed state is to employ bellows means between them. However, the major problem in using bellows is that bellows have a predetermined service life and must be replaced when broken. Disposing such bellows means near the reactor vessel inside a biological shield is not desirable, since a man cannot enter the biological shield when once the operation of the reactor is started.
Another method of fixing the guard vessel to the reactor vessel in a sealed state would be welding. When the guard vessel is to be welded to the outside of the reactor vessel, however, it is not possible to weld all the portions to be welded from both inside and outside of the guard vessel. Namely, welding from outside all the portions of the guard vessel is of course possible, but when the inside of the guard vessel is welded, an exit portion for the welders and welding tools from the guard vessel must be left open and such exit portion cannot be welded from inside of the guard vessel. If there exists any weld portion welded only from outside, the reliability of the seal at that weld portion is lowered remarkably, thus reducing the overall reactor reliability.